Field of the Invention
The invention pertains to a process for operation of a PEM fuel cell unit.
Fuel cells permit direct generation of electrical power from hydrogen and oxygen with much better efficiency and much lower pollutant emissions than ordinary power generators. Moreover, they operate almost without noise.
In addition to these fundamental advantages, the fuel cell with a solid plastic electrolyte (polymer electrolyte membrane (PEM)) has additional positive properties. These properties include a low operating temperature of less than 80.degree. C., favorable overload behavior, limited voltage degradation and high lifetime, favorable load and temperature cycle behavior and the absence of a liquid corrosive electrolyte. In addition, it can also be used for operation with air from the surroundings instead of oxygen.
All of these properties make PEM fuel cells operable with air almost ideal current generators, for example, for exhaust-free electrical operation of vehicles.
PEM fuel cells are not operable by themselves. A PEM fuel cell block, containing a number of PEM fuel cells, an operating part and connected modular electronics are therefore assembled into a PEM fuel cell module. The equipment for supplying hydrogen H.sub.2 and air, for product water takeoff, for removal of waste heat, for moistening of the reactants and for separation of gas pollutants is combined in the operating part.
Important features that characterize the operation of the PEM fuel cell unit with air are the air ratio .lambda. and volumetric air flow rate V.sub.L. The volumetric air flow rate V.sub.L is a gauge of the amount of air flowing through the PEM fuel cell block. The air ratio .lambda. gives the additional air requirement during the reaction, if air from the surroundings is used instead of pure oxygen O.sub.2.
The required dynamics for brief and rapid load changes have proven to be a problem during operation of PEM fuel cell units. An air supply for an air-operated fuel cell system is known from Published, Non-Prosecuted German Patent Application 43 18 818 A, corresponding to U.S. Pat. Nos. 5,434,016, 5,432,020 and 5,645,950, which uses a compressor. The compressor operates according to the displacement principle with a gap-sealed displacer. It also operates oil-free and has a speed spread of at least 1:10. By using a permanently excited synchronous motor, good partial load behavior of the compressor is achieved. However, the Non-Prosecuted Patent Application only describes the components with which this partial load behavior can be achieved.